1. Technical Field
The present application relates generally to the logging of subsurface formations surrounding a wellbore using a downhole logging tool, and particularly to making disparate measurements with the tool while drilling and using those disparate measurements to infer one or more formation properties.
2. Background Art
Logging tools have long been used in wellbores to make, for example, formation evaluation measurements to infer properties of the formations surrounding the borehole and the fluids in the formations. Common logging tools include electromagnetic tools, nuclear tools, and nuclear magnetic resonance (NMR) tools, though various other tool types are also used.
Early logging tools were run into a wellbore on a wireline cable, after the wellbore had been drilled. Modern versions of such wireline tools are still used extensively. However, the need for information while drilling the borehole gave rise to measurement-while-drilling (MWD) tools and logging-while-drilling (LWD) tools. MWD tools typically provide drilling parameter information such as weight on the bit, torque, temperature, pressure, direction, and inclination. LWD tools typically provide formation evaluation measurements such as resistivity, porosity, and NMR distributions (e.g., T1 and T2). MWD and LWD tools often have components common to wireline tools (e.g., transmitting and receiving antennas), but MWD and LWD tools must be constructed to not only endure but to operate in the harsh environment of drilling.
The Schlumberger EcoScope™ tool is capable of measuring formation total porosity (Φ), capture cross section (Σ), and resistivity (R) simultaneously. The collocation of sensors on the tool allows the measurements to be taken substantially in the same formation zone of investigation and substantially at the same time.